Conventional vehicle suspensions employ a spring and shock absorber to isolate wheel motion from body motion. Advances have been made in suspensions by making the suspensions active. For example, in an active suspension system, a controlled force is introduced between the mass of a vehicle body (i.e., a sprung mass) and the mass of a wheel assembly (i.e., an unsprung mass) to achieve improved comfort and handling.
An active suspension system can include an actuator system having a rotary motor disposed on a drive shaft and a transmission mechanism associated with the rotary motor. The transmission mechanism of each rotary motor is typically configured as a ball nut that engages a corresponding threaded portion of the drive shaft. During operation, the transmission mechanism converts the rotation of the rotary motor to a linear motion of the drive shaft along a single direction to adjust a position of the wheel assembly relative to the vehicle body.
In a conventional actuator system, the ball nut of each rotary motor can act as a bearing between the corresponding rotary motor and the drive shaft. For example, each ball nut includes a set of balls carried within a corresponding ball nut channel. The set of balls also engage a threaded portion disposed about an outer periphery of the drive shaft. To minimize binding of the set of balls during operation, the transmission mechanism is configured to allow some movement or play between each of the balls and the ball chamber defined by the ball nut channel and the threaded portion of the drive shaft. However, in the case where the drive shaft and the transmission mechanism change relative translation directions during operation, such as when transitioning from a contracted position to an extended position relative to the vehicle body, the direction of the translational force applied on the drive shaft changes. The change in force can unload one or more of the balls and expose the ball to a zero load crossing condition. As a result, the balls can then float or transition between the ball nut channel and the corresponding thread of the output shaft which generates both force and noise anomalies within the ball nut.